1. Field of the Invention
This invention relates to steam turbines, and more particularly to methods and devices for increasing the power generated by controlling electric charge present in the steam exiting the turbine.
2. Description of the Related Art
In U.S. Pat. No. 5,735,125 Tarelin, et al. reported the presence of positive electric charge in steam exiting the turbine in an electric power generating unit, and disclosed a method to increase the energy conversion efficiency of the generating unit, employing active electrodes in the form of cables stretched above the condenser tube bundle in alternation with grounded but otherwise similar counterelectrodes. When a large positive voltage of appropriate magnitude was applied to the active electrodes, turbulence in the steam flow decreased, and the power generated increased by about one percent.
In U.S. Pat. No. 5,992,152 Weres et al. explained that the charge observed in the steam is the result of electrokinetic phenomena. An electrochemical equilibrium exists between the metal surfaces exposed to steam flow and the thin film of moisture present on the surface, whereby the film of moisture may contain static electric charge of either sign, depending on the acid-base properties of the surface, and the pH and conductivity of the water film. The large gas dynamic shear at the surface of the turbine blades tears the liquid film from the metal surface, and transfers the moisture together with the static charge that it contains to the flowing wet steam. Charge separation may also occur where the film of moisture is torn from the surface of the metal at the trailing edge of a flow guide or baffle plate which is exposed to high velocity steam flow near to the turbine exhaust.
The return of the electric charge from the steam to ground involves electrical discharge activity which drives turbulence in the steam flow and thereby decreases the amount of power generated. U.S. Pat. No. 5,992,152 describes how the amount of electric charge in the steam may be controlled by varying the amount of ammonia added to the feed water, whereby the amount of power generated is increased. The amount of electric charge released to the steam may also be controlled by modifying the composition of the metal surface; for example, applying a stainless steel cladding to the trailing edge of a carbon steel baffle or flow guide exposed to high velocity steam flow will decrease the amount of negative charge released to the steam. This method has several drawbacks: (1) the stainless steel cladding may cause positive charge to be released to the steam (instead of the desired condition of no charge); (2) application of the cladding creates a junction of two disimilar metals, possibly causing localized corrosion; (3) the cladding may protrude from the edge of the plate and interfere with the steam flow.
Depending on the operating regime of the turbine, the charge volumetric density in the turbine exhaust reaches the value of 10xe2x88x927 to 10xe2x88x924 C/m3 and creates an electric field of up to approximately 2xc2x7105 V/m. The electric field associated with the volumetric charge significantly affects the character of the steam flow and increases the pressure after the turbine""s last stage. The electric charge in the turbine steam flow increases the moisture content of the steam by nucleating water droplets, whereby electrical erosion processes are intensified. FIG. 1 shows the location of the plume 1 of maximum electric charge density coming out of the turbine. The plume is approximately cylindrically symmetrical and coaxial with the axis of the turbine, and is located near to the periphery of the last stage (hereinafter xe2x80x9cL-stagexe2x80x9d) of the turbine.
The plume of positive charge coming out of the turbine creates an electric field which pulls the electrically charged steam back toward the turbine, whereby backpressure is increased and the amount of power generated is decreased. In PCT Application PCT/UA99/00019 and U.S. patent application Ser. No. 09/979,255 Tarelin and Skliarov provided a method and device for removing a large fraction of the electric charge from the steam exiting the turbine, whereby the power generated is increased. Stainless steel cables are installed a short distance downstream of the last stage of the turbine and parallel to it. The cables are fastened to the walls of the exhaust hood using high voltage insulators which isolate them from ground and springs which maintain them under tension. The cables are connected to one pole of a DC power supply, and the other pole of the power supply is connected to electrical ground. A voltage of polarity opposite to the sign of electric charge present in the steam is applied to the cables, whereby electrical discharges are generated around the cables, and a substantial fraction of the electric charge is removed from the steam, whereby the power generated is increased. Two arrangements of the electrical cables were provided. In the preferred embodiment, several linear strands of the cable are stretched in a parallel arrangement in a plane parallel to the last stage of the turbine and located a short distance downstream of the last stage. Another installation was suggested, wherein a hexagonal xe2x80x9cspider webxe2x80x9d of cables was stretched and suspended in the same location, in a plane parallel to the L-stage turbine blades.
The method and apparatus described by Tarelin and Skliarov removes a substantial fraction of the charge from the steam whereby the power generated is increased, but has several significant drawbacks: (1) the cables are located in the zones of maximum steam flow velocity, and inevitably interfere with the steam flow, thereby increasing the backpressure and decreasing the power generated; (2) the cables are exposed to intense erosion by the high velocity wet steam flowing past them, and may also be subject to corrosive electrochemical phenomena which will eventually cause them to break; (3) the high velocity steam flow may cause the cables to vibrate, hastening their failure; (4) the high voltage insultors and springs that the cables are attached to are fairly long, and some portion of the insulator bodies may intrude into the zone of high velocity steam flow; (5) the high voltage insulators interfere with the steam flow, and are eroded by it; (6) it may be difficult to actually install this maze of cables, springs and insulators in the location desired relative to the last stage of the turbine; (7) if a cable breaks, it may go flopping around, potentially damaging the last stage of the turbine, and very likely forcing a unit outage to repair the damage; (8) a single cable break may lead to progressive failure of the other cables; (9) a single cable break may render the entire system inoperative by shorting the entire array of cables to ground. For these reasons, the apparatus described by Tarelin and Skliarov in PCT Application PCT/UA99/00019 is impractical, and is unlikely to find industrial application.
The object of this invention is to improve upon the inventions provided in the patents and patent applications cited above in a manner which makes them more suitable for practical application.
The invention disclosed herein is an improved design and arrangement of electrodes to remove electric charge from the steam exiting a steam turbine, whereby the power generated is increased, and the problems with the apparatus that was described in PCT Application PCT/UA99/00019 are eliminated. Instead of cable electrodes stretched across the turbine exhaust, electrodes in the form of thin, sharp-pointed rods (hereinafter called xe2x80x9cpinsxe2x80x9d) are installed around the periphery of the diffuser, or about the periphery of the bearing cone to remove electric charge from the steam exiting the turbine (FIGS. 1, 2 and 3). Similar pins may be mounted on the last stage guide blades to decrease the amount of electric charge released from the guide blades (FIG. 3). Similar pins may also be mounted along the trailing edges of flow guides or baffles that are exposed to high velocity steam flow (FIGS. 4 and 5), whereby the release of electric charge from these edges is decreased, the power generated is increased, and corrosion within the condenser is decreased. The discovery that properly located small, robust electrodes which interfere very little with steam flow can usefully increase the energy conversion efficiency of the generating unit at very low cost was surprising and unexpected. The ability to achieve nearly the same effect using small grounded xe2x80x9cpinxe2x80x9d electrodes was all the more surprising.
Installing pin electrodes around the periphery of the diffuser as illustrated in FIGS. 1, 2 and 3 allows most of the charge to be removed from the steam exiting the turbine while avoiding the problems with the system of cable electrodes that were described by Tarelin and Skliarov in PCT Application PCT/UA99/00019 and U.S. patent application Ser. No. 09/979,255. Specifically: (1) only the pins actually extend into the direct path of steam flow, whereby flow resistance is minimized; (2) the pins are solid rods of stainless steel, which will endure prolonged exposure to the high velocity steam flow with minimal damage; (3) the pins are rigid and relatively short, whereby damaging vibrations will not occur; (4) the collector, insulators, and high voltage cable shown in FIG. 2 are all easily installed behind the lip of the diffuser, and out of the way of the high velocity steam flow; (5) therefore, the collector, insulators, and high voltage cable cannot be damaged by the high velocity steam flow. Similar benefits may be obtained by installing pin electrodes around the bearing cone.
Installing pin electrodes along the trailing edges of flow guides and baffles exposed to high velocity flowing steam as illustrated in FIGS. 4 and 5 is an improvement upon cladding the trailing edges with stainless steel (as described in U.S. Pat. No. 5,992,152) for several reasons: (1) the pins are very small compared to the edge of the plates they are mounted on, and will themselves release very little charge to the steam flowing over them; (2) the length of the junction of two disimilar metals created by installing the pins is small and the junction can be shielded from exposure to the flowing steam by proper design, whereby the probability of local corrosion is minimized; (3) the pins are small and will interfere very little with steam flow, especially if installed along the trailing edges at an angle parallel to the steam flow.